Endogenous proteolytic enzymes serve to degrade invading organisms, antigen-antibody complexes and certain tissue proteins which are no longer necessary or useful to the organism. In a normally functioning organism, proteolytic enzymes are produced in a limited quantity and are regulated in part through the synthesis of protease inhibitors.
A large number of naturally occurring protease inhibitors serve to control the endogenous proteases by limiting their reactions locally and temporally. In addition, the protease inhibitors may inhibit proteases introduced into the body by infective agents. Tissues that are particularly prone to proteolytic attack and infection, e.g., those of the respiratory tract, are rich in protease inhibitors.
Protease inhibitors comprise approximately 10% of the human plasma proteins. At least eight inhibitors have been isolated from this source and characterized in the literature. These include .alpha..sub.2 -macroglobulin (.alpha..sub.2 M), .alpha..sub.1 -protease inhibitor (.alpha..sub.1 PI), .alpha..sub.1 -antichymotrypsin (.alpha..sub.1 Achy), .beta..sub.1 -anticollagenase (.beta..sub.1 AC), and inter-.alpha.-trypsin inhibitor (I.alpha.I).
A disturbance of the protease/protease inhibitor balance can lead to protease-mediated tissue destruction, including emphysema, arthritis, glomerulonephritis, periodontitis, muscular dystrophy, tumor invasion and various other pathological conditions. In certain situations, e.g., severe pathological processes such as sepsis or acute leukemia, the amount of free proteolytic enzymes present increases due to the release of enzyme from the secretory cells. In addition, or separately in other situations, a diminished regulating inhibitor capacity of the organism may also cause alterations in the protease/protease inhibitor balance. An example of such a diminished regulating inhibitor capacity is an .alpha..sub.1 -protease inhibitor deficiency, which is highly correlated with the development of pulmonary emphysema.
In organisms where such aberrant conditions are present, serious damage to the organism can occur unless measures can be taken to control the proteolytic enzymes. Therefore, protease inhibitors have been sought which are capable of being administered to an organism to control the proteolytic enzymes.
One protease that is of particular pharmacological interest is leukocyte elastase. Leukocyte elastase, when released extracellularly, degrades connective tissue and other valuable proteins. While it is necessary for a normally functioning organism to degrade a certain amount of connective tissue and other proteins, the presence of an excessive amount of leukocyte elastase has been associated with various pathological states, such as emphysema and rheumatoid arthritis. To counteract the effects of leukocyte elastase when it is present in amounts greater than normal, a protease inhibitor has been sought which is specific for leukocyte elastase. Such a protease inhibitor would be especially useful if it were capable of being isolated or prepared in a purified form and in sufficient quantities to be pharmaceutically useful.
In the past, at least two leukocyte elastase inhibitors have been identified in the literature. One protein, described in Schiessler et al., "Acid-Stable Inhibitors of Granulocyte Neutral Proteases in Human Mucous Secretions: Biochemistry and Possible Biological Function", in Neutral Proteases of Human Polymorphonuclear Leucocytes, Havemann et al. (eds), Urban and Schwarzenberg, Inc. (1978), was isolated from human seminal plasma and sputum and was characterized as being approximately 11 Kda in size with tyrosine as the N-terminal amino acid. The literature reports of this protein have only furnished a partial amino acid sequence, but even this partial sequence indicates that this protein varies substantially from the proteins of the present invention. The reports of the sequence of this protein, in combination with the complete amino acid sequence data for proteins of the present invention, indicate to the present inventors that the product sequenced by Schiessler et al. may have been a degraded protein which was not a single-polypeptide chain.
A second protein, isolated in one instance from human plasma, has been named .alpha..sub.1 -protease inhibitor. Work on this protein has been summarized in a review by Travis and Salvesen, Annual Review of Biochemistry 52: 655-709 (1983). The reports of the amino acid sequence of this protein indicate that it too differs substantially from the proteins of the present invention.
Because of the substantial differences in structure between single-polypeptide-chain proteins of the present invention and any single-polypeptide-chain serine protease inhibitors of the prior art, the single-polypeptide-chain serine protease inhibitors of the prior art are not "substantially homologous" to the proteins of the present invention.
Trypsin is another protease of particular interest from a pharmacological standpoint. Trypsin is known to initiate degradation of certain soft organ tissue, such as pancreatic tissue, during a variety of acute conditions, such as pancreatitis. A variety of efforts have been directed toward the treatment of these conditions, without marked success, through the use of proteins which it was hoped would inhibit the action of trypsin. Illustrative of such efforts are attempts to use exogenous bovine trypsin inhibitors in treatment of human pancreatitis. While such techniques have been attempted in Europe, they have not been approved as effective by the U.S. Food and Drug Administration. Thus, there is a need for a protease inhibitor effective in neutralizing excess trypsin in a variety of acute and chronic conditions. As was the case with the leukocyte elastase inhibitor discussed above, a trypsin inhibitor would be particularly useful if it could be isolated and prepared in a purified form and in sufficient quantities to be pharmaceutically useful.
Cathepsin G is another protease present in large quantities in leukocytes. Cathepsin G is known to be capable of degrading in vitro a variety of valuable proteins, including those of the complement pathway. Pancreatic elastase is another protease which may have a role in pancreatitis. Thus, inhibitors for these proteases are also of potential pharmaceutical value.
Leukocyte elastase, trypsin, cathepsin G and pancreatic elastase are examples of a class of proteases known as serine proteases, which have elements of common structure and mechanism. Their activity against different substrates and their sensitivity to different inhibitors are believed to result from changes in only a few amino acid residues. By analogy, it is possible to conceive of a class of serine protease inhibitors, also having common elements of structure and mechanism, in which changes in a relatively few amino acids will result in inhibition of different proteases, and that at least one member of this class will inhibit it every serine protease of the former class. The class of serine protease inhibitors would then be of substantial value.
Surprisingly, the present inventors have found a 12 Kda protease inhibitor, isolated in purified form from parotid secretions, in which the amino acid sequence varies greatly from the reported sequences of single-polypeptide-chain serine protease inhibitors. The protease inhibitor of the present invention is believed to have at least two active sites. One site exhibits leukocyte elastase inhibiting properties while a second site exhibits activity against trypsin. The present inventors have accurately sequenced the entire length of the novel protease inhibitor. This sequence is set forth more fully hereinafter.